1. Field of the Invention
The present invention relates to a method of reading the optical image of an inspected surface and an image reading system employable in an optical appearance inspecting apparatus for a printed circuit board etc. More particularly, the present invention relates to improvement for increasing incident light intensity in an image sensor equipped in the image reading system.
2. Description of the Background Art
As is well known in the art, a printed circuit board is provided with a metal wiring pattern on its one or each surface, while a through hole for accepting the lead wires of an electronic component is formed in a direction penetrating the circuit board. In order to inspect whether or not the wiring pattern and the through hole are formed with an accuracy within a predetermined tolerance, various types of optical appearance inspecting apparatuses are employed.
FIG. 27 is a conceptual diagram showing a conventional image reading system employed in a wiring pattern inspecting apparatus. Light 2 emitted from a light source 1 is reflected by a half mirror 3, and applied to the surface of a printed circuit board 5. The printed circuit board 5 is provided with a wiring pattern 6 and a through hole 7, and reflected light 8 obtained by reflection of the light 2 on the surface thereof is imaged on a linear image sensor 9 through the half mirror 3 and an imaging lens 4.
FIG. 28 shows exemplary incident photo-levels in the linear image sensor 9, which photo-levels correspond to a linear image along the line K--K in FIG. 27. The wiring pattern 6 has a large light reflectance since it is made of a metal, and an incident photo-level corresponding to the wiring pattern 6 is also large. On the other hand, an incident photo-level from an insulating base 5a of the circuit board 5 is relatively small, while the light 2 is transmitted through the through hole 7 downwardly from the circuit board 5 and the photo-level therefrom is substantially zero. Thus, it is possible to obtain the image of the wiring pattern 6 by discriminating the respective incident photo-levels using a threshold value TH.
The conventional apparatus shown in FIG. 27 is based on the premise that differences between the photo-levels from the respective regions 5a, 6 and 7 are large. However, the light reflectance on the surface of the wiring pattern 6 is not necessarily uniform, and the photo-level therefrom may fluctuate. Further, the waveforms of the photo-levels are complicated by reflected light from an inner wall portion of the through hole 7, so that the boundary between respective images of the wiring pattern 6 and the through hole 7 is confused.
In order to cope with such circumstances, there has been proposed such a system that another light source is provided on a back surface side of a printed circuit board and the image sensor detects not only the reflected light but also the light which is transmitted through a through hole from the light source newly provided. For example, Japanese Patent Publication Gazette No. 62-29737 (1987) discloses such a technique. Further, Japanese Patent Laying-Open Gazette No. 62-276443 (1987) discloses an apparatus for catching only a hole image.
However, these prior art methods have the following disadvantages (1) and (2).
(1) In the apparatus shown in FIG. 27, only half the intensity of the light 2 from the light source 1 is reflected by the half mirror 3 and directed to the printed circuit board 5. Further, only half the light 8 reflected by the printed circuit board 5 transmits through the half mirror 3 to reach the image sensor 9. Even if the light reflectance at the wiring pattern 6 is 100%, therefore, the intensity of light reaching the image sensor 9 is 1/4 of that of the light that leaves the light source 1.
Thus, the intensity of the light received in the image sensor 9 is relatively small, and image detection accuracy is not necessarily high. This is a common problem not only in an apparatus employed for appearance inspection of a printed circuit board, but also in appearance inspecting apparatuses as to various inspected objects.
(2) In optical appearance inspection, it is necessary to independently grasp respective states of formation of the wiring pattern and the through hole, while interpositional relations between the wiring pattern and the through hole must also be inspected. Thus, there is required an inspecting apparatus, which can simultaneously catch respective images of the wiring pattern and the through hole. However, the aforementioned prior art methods are merely adapted to catch the image of only one of the wiring pattern image and the through hole image, and is not structured to simultaneously grasp both images. Therefore, when such a prior art approach is employed, it is necessary to separately detect the respective images in order to recognize interrelation between the position of the wiring pattern and that of the through hole and then calculate the positional relation between the two images. Therefore, the inspection time is increased and the system structure is complicated.
Such circumstances are particularly aggravated in the system in which the diameter of through holes provided in printed circuit boards is so gradually reduced that through holes (mini via holes) of 0.5 to 0.1 mm in diameter, for example, are employed.